That’s the No. 1 question from corn producers these days, according to Glen Harris, University of Georgia Extension soil scientist.

“The easy answer is ‘a lot,’” he says. “The University of Georgia does have recommendations to take you to that 300-bushel yield. It reminds me of what happened with cotton when we used to have the old recommendation of 750 pounds per acre — now we go up to three bales.”

Harris discussed corn fertilization during the recent Corn Short Course and Georgia Corn Growers Annual Meeting held in Tifton.

For corn in Georgia, whenever you drop off your sample, irrigated base rates are 150 bushels per acre, says Harris.

“But if you read the fine print, for every 10-bushel increase, you add 12-6-10, as far as N, P and K. So if you look at nitrogen, it’s pretty straight-forward.

“For 150 bushels, we recommend 180 pounds of nitrogen per acre. For 200 bushels, you go up to 240 pounds, and for 250 bushels, go to 300 pounds. For 300 bushels per acre, we recommend more than 360 pounds of nitrogen per acre,” he says.

For P and K, it makes a difference where you’re starting from — low, medium or high, says Harris. “With a 300-bushel goal, even when you’re already high in P and K, we’re still recommending a high amount of P and K to feed that crop. Some of that is for maintenance,” he says.

But that’s only the fertilizer rate, he cautions growers.

“The latest buzz for those who work with fertilizer is not only rate but also source, timing and placement, which all come into play. You can’t think that if you put out 360 pounds of nitrogen, you’ll automatically make 300 bushels of corn.

“There are a lot of other limiting factors such as water, weed control, disease control and others, so just keep that in mind. We tend to focus on nitrogen and water as the primary limiting factors in corn,” says Harris.

Starter fertilizer

Turning to starter fertilizer in corn, Harris says he’s looking in his research at potassium in the furrow. Growers traditionally use small amounts of nitrogen and phosphorus as starter fertilizer.

“Our data showed that we actually did get more potassium into the plant when we put it in the starter, but it didn’t translate into yield,” he says.

“We’re pretty comfortable now with 10 gallons of 10-30-40 in the furrow on corn, after two years of looking at it, although it might not always translate into yield. Even with no nitrogen in the furrow and potassium alone, we still did okay.

“As long as you account for other nutrients in your program, things like 10-30-40 and some nitrogen solution is not a bad way to go.

“It didn’t hurt the stand in corn. It’s one of the concerns we have with putting stuff in the furrow. I’m still not comfortable with putting anything in the furrow with cotton. Corn is a hardier seed, and the in-furrow treatment did not hurt stand.”

The main advantage of starter fertilizer is better early-season growth, according to University of Georgia Extension recommendations.

Corn planted in February, March or early April is exposed to cool soil temperatures, which may reduce phosphate uptake.

Banding a starter fertilizer 2 inches to the side and 2 inches below the seed increases the chances of roots penetrating the fertilizer band and taking up needed nitrogen and phosphorus.

Harris says he also conducted a simple phosphorus rate study this past year.

“I found a really low soil-testing phosphorus patch on the experiment station that had a phosphorus level comparable to what you’d normally see when you’re going into new ground.

“The recommendation for 200-bushel corn is actually 140. We say that if you have medium-testing phosphorus soils, you only have a 50-percent chance of seeing yield response from adding it. But you don’t need to take that chance — go ahead and put it out there.”

Of the 16 essential plant nutrients, there’s a lot of focus on N, P and K, but micronutrients can limit yield as well, says Harris. “They’re needed in very small amounts by the plant, and they also have a very narrow window of sufficiency range in the plant. In corn, we concentrate on boron, manganese and zinc.”

Zinc deficiency can be prevented by using thee pounds per acre of actual zinc. Do not use zinc unless soil test levels are low. If needed, apply preplant or at planting.

Boron deficiencies can occur on sandy soil low in organic matter. Generally, use one to two pounds per acre of boron applied in split applications. It is best to apply boron with the nitrogen applications.

The application of other essential nutrients should be based on plant analysis results.